1. Field of the Invention
This invention pertains to optical waveguides and detachable connectors therefor, with particular application to hermetic packages housing an optical source or sink and detachable connectors that mate with features of the package.
2. Description of the Related Art
Optical or lightwave communications are being utilized in more applications today than ever before. These optical systems provide benefits not previously available in electrical systems. Benefits such as large bandwidth available for signal transmission, excellent noise immunity, limited crosstalk, and low weight per unit bandwidth are common.
However, demanding applications for optical systems continue to motivate further development in several areas where optical systems are more restrictive than electrical counterparts. Chief among the disadvantages of an optical system are the relative fragility of the glass fibers used as lightwave conductors and the difficulty interconnecting these fibers to transmitters, receivers, and other fibers. Low-loss, low cost, reliable and easily removable connectors are very much needed to continue to expand the applications for optical technology.
There are several factors that determine the cost, quality and applicability of an optical connection. The fragility and small size of a typical fiber is important. During manufacture production costs escalate while quality and yield suffer if the fiber is damaged. During installation an installer will need to make all needed interconnections without adversely affecting the fiber or the connection. A fiber should not be scratched or bent excessively, and generally is jacketed to prevent this type of damage. However, this objective of ease of handling during manufacture and installation is usually in opposition to the advantages provided intrinsically by the fiber of low size and weight for a given bandwidth capacity.
The efficiency of the coupling from the fiber to an optical source or sink is also very important. For the purposes of this disclosure an optical source or sink is defined to be an LED, laser diode, phototransistor, attenuator, modulator, optical IC, waveguide, polarizer, filter, other optical fiber or similar component which by design generates, transmits or receives a lightwave. Coupling efficiency is often described as insertion loss. Insertion loss in axial type interconnections can result from center to center mismatch between the source and sink axes, angular misalignment of the source and sink axes, separation between the source and sink, optical imperfections on the surfaces of or in between the source and sink, and reflections. For two fused silica fibers having a diameter of 5 mils, center to center mismatch of only 0.5 mils will add a loss of approximately one db to the connection loss. Similarly, angular misalignment of about 3 degrees will also add about one db of loss.
Any foreign matter between the fibers may also adversely impact insertion loss. Where the fiber is connected through a package, package hermeticity may also be important. For purposes of this disclosure, a hermetic seal is defined as one which provides a very low leak rate, in a range comparable to that achievable with a fully brazed entirely metal package, and which also provides a lack of atmospheric exchange as would be detected by a residual gas analysis (RGA).
Typical polymer materials are not able to meet the long term hermeticity and RGA requirements due to infusion of moisture and other contaminants. Leak rates less than 1.times.10.sup.-8 atm-cc/second are commonly obtainable with metals and non-porous glasses and non-porous ceramics. Metals, non-porous glasses, and non-porous ceramics will also meet the RGA requirement imposed for the purposes of this disclosure, while, as noted, known polymers will not.
Moisture impervious seals and gas-tight seals are defined for the purposes of this disclosure as seals that provide an environmental barrier to infusion of water, dirt, and other contaminants present with rapid infusion of air. This type of seal is typically obtained with a polymer type seal ring, and may be non-hermetic as defined herein.
The space required for an interconnection may prevent some otherwise successful designs from being accepted for a given application. The relative ease of attachment of the connector is also very important. Advantages gained in a fine fiber with low mass are lost by a bulky connector and heavy fiber reinforcement. Similarly, if the fiber is for all practical purposes permanently affixed, perhaps due to difficulty in alignment, any benefit gained by a detachable connector may be lost.
Optical connectors often use some type of sleeve to align fibers both center to center and axially. Examples of such alignment sleeves may be found in U.S. Pat. Nos. 4,193,665, 4,636,034, 4,707,068, 4,750,803, 4,892,379, 4,948,224, 5,000,537, and 5,005,940, hereby incorporated by reference. These sleeves may be expandable and slide directly around the fiber, but more often a ferrule is used to carry the fiber. The ferrule will provide a more mechanically sound structure for insertion into and removal from the sleeve, while generally preventing any scratching from occurring at or near the optically active surfaces of the fiber.
In addition to ferrules and an alignment sleeve, fiber butt connectors require mechanical retention to maintain the fibers in a closely spaced relationship. Once again, there is a size and performance trade-off demanded by the prior art. A physically strong connector which is resistant to flexure and pull of the fiber will usually require a large latching and spring structure to maintain the fibers closely spaced and aligned. Examples of these latching mechanisms may be found in any number of U.S. Pat. Nos., such as 4,140,365, 4,553,813, 4,737,008, 4,747,656, 4,762,388, 4,762,389, 4,779,952, 4,799,758, 4,877,303, and 4,896,938, also incorporated herein by reference.
The present invention seeks to overcome those prior art limitations of size and weight while still retaining the advantages of low insertion losses, low weight per unit bandwidth, high bandwidth capacity, and detachable interconnection.